This disclosure relates to a method and apparatus to oppose a short circuit failure mechanism in a printhead drive circuit of a printer, and to provide a means to distinguish between failed and non-failed printheads.
It is common for an inkjet printer to have both a black printhead (pen) and also a tri-color pen. By providing two pens, either color or monochromatic documents may be printed. To lower costs, it is common for a single control circuit to be used in the management of both pens. Such a circuit, commonly including a single integrated circuit (IC) known as a head driver, is typically contained within a central location and connected to each pen. Address lines, controlled by the head driver, extend into both pens, allowing any nozzle within either printhead to be selected. Following selection of a nozzle address, a pulse of current may be sent to the desired nozzle using black or color xe2x80x9cprimitives,xe2x80x9d i.e. wires which transmit a pulse of current to firing resistors associated with the nozzle selected by the address lines.
A failure mechanism associated with such printheads results when ink becomes located in a position that results in a short circuit, typically between an address line and ground. Because of the electrical conductivity between the address lines between at least two printheads, when an address line is shorted to ground in one printhead, a plurality of printheads may fail to operate. Moreover, it is difficult to determine in which printhead the short circuit is located. As a result, the printer is completely disabled, and repair costs are increased.
An engineering solution that is used, and which provides immediate information on the location of the short circuitxe2x80x94i.e. the printhead containing the short circuitxe2x80x94is to provide a head driver IC or an entire control circuit for each pen. This solution also allows the non-disabled head to operate. Unfortunately, this results in greater cost, due to the need for two head driver ICs.
Accordingly, a solution is needed to cope with a failure within a first printhead, which allows other printheads within the printer to operate. The solution should not require a second head driver IC, and should allow easy discovery of the identity of the printhead in which the failure occurred.
In a typical inkjet printer configuration, one black and one tri-color printhead are provided. In such a configuration, address lines in communication with a single head driver IC extend into both printheads. A failure protection circuit associates one resistor group of address line resistors with each address line. In one implementation, one address line resistor is placed in series between the address lead extending from the head driver IC and each printhead into which the address line extends.
A common failure mechanism results when ink within one of the printheads causes a short, tying one of the address lines to ground. Without the failure protection circuit, the conductivity of the address lines between two or more printheads would cause a failure in all printheads. However, the address line resistor within the failure protection circuit, associated with both the shorted address line and the printhead wherein the failure occurred, begins to function as a pull-down resistor. Accordingly, the head driver IC is able to control the voltage potential of that address line in each printhead wherein no failure was experienced. The user is then able to identify and replace the non-functioning printhead.